Define meaningful tool frame

When a hand is mounted, the link8 frame is 45° rotated wrt. the hand.
Better define a tool frame that is nicely aligned with the hand, i.e.
x=normal, y=slide, z=approach, and nicely centered between the fingers.

franka_description/robots/panda_arm.urdf.xacro

@@ -18,6 +18,21 @@
     </xacro:if>
   </xacro:unless>
 
+  <!-- Define a tool frame -->
+  <link name="$(arg arm_id)_tool" />
+  <joint name="$(arg arm_id)_tool_joint" type="fixed">
+    <xacro:if value="$(arg hand)">
+      <!-- Place between the fingers, orientation: x=normal, y=slide, z=approach -->
+      <origin xyz="0 0 0.1034" rpy="0 0 ${-pi/4}" />
+    </xacro:if>
+    <!-- Identical to link8 -->
+    <xacro:unless value="$(arg hand)">
+      <origin xyz="0 0 0" rpy="0 0 0" />
+    </xacro:unless>
+    <parent link="$(arg arm_id)_link8" />
+    <child link="$(arg arm_id)_tool" />
+  </joint>
+
   <xacro:if value="$(arg gazebo)">
 
     <xacro:arg name="xyz" default="0 0 0" />